The currently utilized polarizers represent polymer films oriented by uniaxial stretching and colored with organic dyes or iodine compounds. Polyvinyl alcohol is usually used as the polymer (PVA) in these films [U.S. Pat. No. 5,007,942]. Additional layers in such a polarizer, for example, a layer of lacquer, perform protective and other functions and are not intended for optimization of transmission of light through the polarizer (or reflection of light—for a reflecting polarizer) from the point of view of light interference.
There is a known polarizer [WO 95/17691], which represents a multilayer structure comprising at least one birefringent layer with the thickness, which provides interference extremum at the exit of the optical polarizer, for at least one linearly polarized component of light. This polarizer comprises alternating layers of two transparent (non-absorbing in the working range of wavelengths) polymer materials, one of which is birefringent, and the other—optically isotropic. The birefringence in the polymeric material is obtained via stretching it in one direction by 2–10 times.
The working principle of such polarizer is in the following: one linearly polarized component of light, to which corresponds the extraordinary (larger) refraction index of the birefringent layer, is significantly reflected from the multilayer optical polarizer due to the difference in the refraction indexes at the boundaries between the isotropic and the anisotropic layers. With the corresponding choice of the layers' thickness and refraction indexes the optical path difference between the wavelengths reflected from the boundaries of layers comprises a whole number of wavelengths, i.e. there is interference maximum. In this case, reflection of the linearly polarized component of light, to which corresponds the extraordinary (larger) refraction index of the anisotropic layer significantly increases.
The ordinary (smaller) refraction index of the anisotropic layer is chosen close to the refraction index of the isotropic layer, therefore the other linearly polarized component of the incident light, to which corresponds the ordinary (smaller) refraction index, travels through the multilayer optical polarizer without reflections.
Thus, upon incidence of an unpolarized light onto the known polarizer, one linearly polarized component of light is reflected, while the other passes through practically without losses (in essence, the polarizer functions as a beam splitter).
Multilayer polarizer may also contain additional dichroic polarizer (with weak absorbance), axis of which is parallel to the axis of the reflecting polarizer. The role of the dichroic polarizer is basically to remove reflections of the external light when such combination polarizer works “in transmission”.
One of the drawbacks of the known multilayer polarizer is the necessity to use large number of alternating layers, due to the low degree of anisotropy (the deference between the ordinary and extraordinary refraction indexes) in the transparent polymer materials. Usually this value does not exceed 0.2. Therefore, coefficient of reflection from the boundaries of layer is small, and to obtain high coefficient of reflection in general one needs 100–600 layers, deposition of which poses difficult technical challenge and requires special precision equipment.
There is a known polarizer [WO 99/31535], comprising at least one anisotropically absorbing layer, at least one refraction index of which increases with the wavelength. To increase effectiveness of light polarization, it has been suggested to use multilayer polarizer, in which the thickness of layers and their refraction indexes are selected such as to provide interference extremum for at least one linearly polarized component of light. This is the so-called “interference-type” polarizer. Anomalous dispersion of at least one refraction index of the anisotropic layer allows to effectively polarize light in a wide spectral range. In the capacity of the material of the anisotropic layer, it has been suggested to use corresponding dichroic dyes, their mixtures and dyes with various modifying additives. The degree of anisotropy of these materials is significantly higher, than of the stretched polymer films. However, the modern level of technological development, in particular displays, requires obtaining coatings with higher degree of anisotropy and perfect structure.
Patent [RU 2155978] describes polarizers comprising a film of dichroic organic material, molecules of which or fragments of molecules of which have flat structure and at least a part of the film has crystalline structure. In particular, such films may be fabricated from various dyes and their mixtures. Crystalline structure of these films allows obtaining high degree of anisotropy and homogeneity of optical characteristics.
However, experiments have shown that such films are hygroscopic and require additional protection or processing to modify their chemical properties. For this purpose, in particular, the already finished films are processed with ions of the 2x and 3x valence metals.
Optical characteristics of the known films are determined by the order parameter, which in this case is the averaged characteristic, not accounting for the particular situation of the optical axes of the crystalline structure relative to the substrate, which in turn, imposes restrictions on the possibility to obtain structures “film-substrate” with the given optical properties.
Other literature also describes other multilayer structures with an optically anisotropic layer, having other (not limited only to the polarizing light) functional purpose. For example, interference-polarizing (IFP) light filters, operation of which is based on interference of the polarized rays [Gvozdeva et al., Fisichaskaya optika, M.: Mashinostroenie, 1991]. The peculiarity of these filters is in the possibility to extract very narrow spectral ranges (up to 10−2 nm) without any background noise. Often to fabricate separate layers of IFP, one uses thin plates of various crystals, for example crystalline quartz or Iceland spar.